With the use of digital signals, squarewave signals and a multitude of other variety of high frequency signals, it is often necessary that a transmission line maintain a uniform impedance characteristic in order that digital signals or high frequency signals transmitted along the line are not distorted, reflected or altered in phase when the transmission line is carrying or conveying the signals. Certain digital data processing systems, data communications systems, testing systems and other switching systems using high frequency signals require that the input end of a transmission line be routed over to a variety of other output terminals after being switched through different paths.
In order to keep the disturbance to the line as small as possible, this requires a switching system with a controlled impedance matching the impedance of the transmission line and maintaining the constant impedance no matter how many switches are in the line. Generally, these types of transmission line systems are designed to have a characteristic impedance somewhere between 50-100 ohms, which theoretically remains constant at the designed impedance; however, any changes in length or routing may lead to problems in high frequency switching systems if the impedance-constancy of the transmission line is not maintained.
One place in the digital processing field where the problem of line variance arises is in the testing of high speed integrated circuits. In this situation, pulses with fast edges rates (high-speed rise-time and fall-time) are used. These types of pulses contain a wide range of frequencies which complicates the problem of switching the signal through various other transmission lines without introducing distortion to the signal pulses.
The problem of maintaining a constant impedance transmission line and still permitting switching of the line into different output paths has heretofore been handled by the use of so called "coaxial relays" which are precisionally built and extremely expensive in cost per unit. These coaxial relays can switch an input signal to one of two (sometimes more) outputs. These generally are well designed and introduce very little distortion to the input signal travelling through.
Generally, when switching systems have to be applied to computer applications, such as the testing of high speed integrated circuits, these may involve the connecting of hundreds of terminals to be tested. Since a multitude of coaxial relays are required, the cost of building a test system using these coaxial relays becomes prohibitive. For example, a test system (which may routinely need to switch an input signal to one of 50 available output terminal lines) would require such a multitutde of coaxial relays, to do the job properly, that it would make this application unreasonably expensive. Thus, commercial systems which would require the use of this type of switching with coaxial relays find that it is prohibitive.
A number of manufacturers have attempted to handle this problem in the fashion shown in FIG. 1 by connecting many small single throw relays 11.sub.o to one point and closing only one relay at a given time. Generally, these relays are the small inexpensive reed relay types. However, a persistent problem that arises with this solution, as seen in FIG. 1, is that each relay connected to a common node 9 will form a stub and add a capacitative load or discontinuity. For example, in FIG. 1, while one relay is closed, there are three other open relays hanging on or connected to the common node 9 which introduced a set of stubs making a capacitative load. In FIG. 1, for example, if the transmission system is designed as a 50 ohm system, which is customary in IC test systems, each relay is made as a "50 ohm" relay. This means that such a closed relay will behave electrically as if it were a piece of 50 ohm transmission line. However, each "open" relay connected to the node 9 behaves as a short stub. Thus, the only way to reduce the amount of load capacitance at that node is either to reduce the size of the relay or to reduce the number of relays, which again tends to defeat the problem of switching a transmission line. This cluster-of-relays approach in FIG. 1 can never be a truly controlled-impedance system.
The practical answer to the problem is provided by the use of small inexpensive single-pole double-throw reed-type relays which can be placed on or embedded in a printed circuit board. The preferred apparatus described herein uses a single-pole double-throw reed relay which has one input and two output lines and does not have an off position. The input line is always connected either to one or the other of the two output lines. This should be contrasted with the "single throw" type relays which have one "off" position and one "on" position only. One typical type of the preferred reed relay is designated as a Form C Reed Relay as manufactured by Hamlin, Inc., Lake and Grove Streets, Lake Mills, Wis. 53551. Another similar type of such relays are the miniature mercury-wetted relays known as Log-cells such as manufactured by Fifth Dimension, Inc., Post Office Box 483, Princeton, N.J. 08540.